March 1995, Volume 2

Recently, a Power Plant Superintendent asked us to evaluate the practicality of modifying his F.D. fan for increased capacity. Since his fan was motor driven with actual amp readings at 85% of rated name plate values, we advised him that an increase in flow rate of 5% was possible by tipping the fan rotor and utilizing the full capacity of his motor.

The 5% capacity increase is calculated from a fan law which defines CFM as a cube root function of horsepower. Any increase in flow cannot be achieved unless the fan pressure capability is also increased; thus, 11% more pressure is required to force 5% additional flow through the existing F.D. fan system. (Pressure is a function of flow squared.)

In this case, the superintendent stated a capacity increase of 5% is not enough so he asked for a proposal to replace the motor and complete fan assembly. Because of prohibitive cost, this action was quickly deemed inappropriate.

Rather than conclude that no action was possible, an engineer from Process Equipment Company asked to visit the plant site. With two instruments, a thermometer and an amp meter, our engineer quickly calculated two important parameters which are necessary to evaluate any action to increase flow rate:

With the thermometer, the fan efficiency was evaluated to be very low. With the combination of thermometer and amp meter, the flow rate at the fan was also defined and determined to be considerably less than what was perceived from evaluating the fan performance curve and other boiler data.

The low fan efficiency was attributable to the original equipment manufacturer's fan wheel design which appeared to be excessively wide. A dissertation involving turbo machinery design would be required to explain this fact; a subject matter which will be addressed in a future newsletter. In this particular instance, the field data that was measured unquestionably defined fan efficiency as the culprit. Knowing this fact, our engineer again evaluated the possibility of a replacement fan. Since the efficiency of the replacement fan would be considerably higher than the existing fan, the existing motor could be reused and the cost of just the fan was considered an affordable solution to the problem.

Listed below is a fundamental fan formula which explains the relationship between volume, static pressure and fan efficiency. With fan efficiency in the denominator, ant improvement in fan efficiency significantly allows the fan to develop more volumetric flow rate at greater head without increasing horsepower requirements.

HP=(CFM x Pressure Rise) / (6356 x Fan Efficiency)

In conclusion, the significance of this case history is that a prerequisite to a fan performance upgrade is the determination of existing fan efficiency which a knowledgeable fan engineer can determine utilizing a simple thermometer and electrical amp meter. If further information is desired about this procedure. Process Equipment can mail a documented paper which was presented at an EPRI Conference, which outlines procedures for field measuring volumetric flow rate as mentioned above.

Overfire Air Fan Systems
Because of the new air pollution control cluster rules, we are seeing even more requirements to lower individual stack emission rates. It is not always necessary to address this problem by investigating the possibility of replacing or modifying air pollution control equipment. Often stack emissions can be lowered by optimizing the source of the emission such as in the furnace of a wood fired boiler. Process Equipment has considerable expertise in this field.

Process Equipment Company designs replacement overfire air systems. The results of these modified overfire air systems can be lower stack emissions and also increased wood burning capability.

For additional information on articles appearing in this newsletter, contact Process Equipment Company at (205) 663-5330 or E-mail to: information@process-equip.com


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